Flow through ion selective electrode

ABSTRACT

An improved flow through ion sensing electrode and a method for producing it. The electrode is formed of bilumen plastic tubing using one lumen as the sample path and the other lumen as a sensing chamber. By permeating a portion of the tubing with an ion selective material, an ion selective membrane integral with the chamber wall is produced. A preferred embodiment also incorporates a reference electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned copending patent application Ser.No. 519,937 by Cahalan, Fogt and Jevne for "Ion Selective Membranes ForUse In Ion Sensing Electrodes" and to commonly assigned copending PatentApplication Ser. No. 519,938 by Cahalan and Schwinghammer for a"Combination Ion Selective Sensing Electrode", both filed Aug. 3, 1983.

BACKGROUND OF THE INVENTION

This invention relates to electrodes for measuring ion concentrations inaqueous solutions and to methods of manufacturing such electrodes andthe ion-selective membranes they employ.

Ion-sensing electrodes selectively responsive to ionic activities inaqueous solutions are well known to the art. The known relationshipbetween ionic activity and ion concentration permits these electrodes tobe used to measure ion concentrations. In such electrodes, selective ionexchange occurs through an interface between a selective ion exchangematerial and the solution to be samples. Many recent electrode designshave employed ion-selective membranes which retain the ion-selectivematerial in a matrix of organic material such as polyvinylchloride orother plastic.

Many prior art sensing membranes employing a plastic matrix are preparedby dissolving the ion-selective material in a solvent which either is,or has dissolved within it a plasticizer for the plastic. The plastic,in powdered form, is added to this mixture. The membrane is cast in itsdesired form by applying this mixture to a substrate having the desiredshape and evaporating the solvent or otherwise curing the mixture.Examples of such electrodes are described in U.S. Pat. No. 3,450,631issued to Bloch, et al, U.S. Pat. No. 3,635,212, issued to Watanabe,U.S. Pat. No. 4,271,002 issued to Hawkins and U.S. Pat. No. 4,242,191issued to Schindler et al.

In some prior art electrodes, the substrate on which the ion-selectivemembrane is formed is incorporated as part of the electrode structure.In others, the membrane is removed from the substrate and attached tothe body of the electrode. In such electrodes, sealing of the membraneto the remainder of the electrode structure has been problematic.

U.S. Pat. No. 4,233,136 issued to Spaziani, et al, addresses the problemof sealing the ion-selective membrane to the remainder of the electrodestructure, in the context of incorporating an ion-selective membrane inthe wall of a plastic tube. This method, like that of the prior artelectrodes discussed above, involves dissolving powdered plastic in avolatile solvent containing a plasticizer and an ion-selective materialand evaporating the solvent to form the membrane. In the Spazianielectrode, a plastic tube is provided with a lateral opening, and acylindrical mandrel is inserted in the tube across the opening. Themixture is applied to the mandrel, filling the opening of the tube. Thevolatile solvent fuses the membrane to the plastic tube. After themembrane has formed, the mandrel is removed leaving the membrane inplace.

SUMMARY OF THE INVENTION

The present invention includes a novel method of producing anion-selective membrane and a novel method of producing an ion-sensingelectrode employing an ion-selective membrane of a desired shape. Themethod for producing an ion selective membrane comprises soaking aplastic member of the desired shape in a solution of an ion-selectivematerial dissolved in a volatile solvent which is a swelling agent forthe plastic of which the member is fabricated, and drying the member toremove the solvent. By this method, a tubular structure including anion-selective membrane, for example, may more easily be produced. Thismethod has as a major advantage that it allows the use of pre-formed,commercially available plastic products, such as single and bi-lumentubing, to easily and simply fabricate a variety of ion-sensingelectrodes. Because the plastic member is first formed to the desiredshape and subsequently treated to transform a portion of the member intoan ion-selective membrane, the membrane is continuous with and integralto the member, and the problem of sealing the membrane to the electrodecan be avoided. Further, because this method does not require use of asubstrate to form the ion-selective membrane, production of the membranein any desired shape is substantially simplified.

The present invention also includes the use of the above process tofabricate ion-sensing electrodes from commercially available plasticmembers. The methods of fabricating these electrodes are substantiallysimplified as compared to the methods used to fabricate prior artelectrodes. In addition, electrodes formed using the methods set forthherein are believed superior to prior art electrodes in that theirion-selective membranes are integral to the electrode structures.

One preferred embodiment of an electrode according to the presentinvention uses single lumen polymer tubing to produce a simpleion-sensing electrode. Other preferred embodiments employ bi-lumentubing to produce combination ion-sensing and reference electrodesadapted to be dipped into the sample solution. Yet other preferredembodiments employ bi-lumen tubing to easily produce ion-sensing andcombination-sensing and reference flow through electrodes. The presentinvention allows all of the described embodiments to be easily producedrelative to prior art electrodes. In addition, each described embodimenthas unique and valuable features that are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a tubular plastic member, appropriate for use to produce anion-selective membrane according to the present invention.

FIG. 2 shows an ion-sensing electrode incorporating an ion-selectivemembrane fabricated of the member of FIG. 1.

FIG. 3 shows a combination electrode employing a membrane according tothe present invention, fabricated of a bi-lumen tube.

FIG. 4 shows an alternative combination electrode, also employing amembrane according to the present invention, fabricated of a bi-lumentube.

FIG. 5 shows a flow-through ion-sensing electrode employing a tubularmembrane according to the present invention.

FIG. 6 shows an alternate flow-through ion-sensing electrode employing amembrane according to the present invention, fabricated of a bi-lumentube.

FIG. 7 shows a combination flow-through electrode employing a membraneaccording to the present invention, also fabricated of a bi-lumen tube.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a tubular plastic member 10 having lumen 12, appropriatefor use in fabricating an ion-selective membrane according to the methodof the present invention. Member 10 may be fabricated of siliconerubber, polyvinylchloride, polyurethane, or other swellable plastic. Inorder to transform member 10 into an ion-selective membrane, thefollowing steps must be performed. First, member 10 must be soaked inwhole or in part in a solvent in which the desired ion-selectivematerial has been dissolved and which acts as a swelling agent for theplastic of which member 10 is constructed. Soaking should continue untilmember 10 has swelled, indicating incorporation of the ion-selectivematerial. Subsequently, the solvent is removed, leaving behind theion-selective material and allowing the swelling of member 10 tosubside. Preferably, the solvent used should be volatile so that it maybe conveniently removed by evaporation.

FIG. 2 illustrates a side cross sectional view of an ion-sensingelectrode according to the present invention. The ion-selective membraneof the electrode is fashioned from plastic tube 20, which has outer wall22 impregnated with an ion-selective material, as described above. Thelumen 36 of tube 20 is filled with an electrolyte solution or gel 26.The distal end of tube 20 is sealed with plug 24, which may be made ofsilicone rubber or other suitable material. The proximal end of tube 20is sealed by connector block 30, which may be made of epoxy or othersuitable material. Mounted within connector block 30 is connector pin 32which is crimped to the proximal end of electrode 28. Electrode 28 maybe a silver/silver chloride reference electrode, well known to the art.Connector pin 32 may be made of any conductive metal such as stainlesssteel. Electrolyte 26 may be any suitable electrolyte solution or gelcontaining chloride ion and the ion to be detected.

The above electrode is easily fashioned by first treating tube 20 asdiscussed in conjunction with FIG. 1 above. The entire tube may betreated, or only a portion. For example, only the portion distal todotted line 34, may be dipped in the solvent. Following evaporation ofthe solvent, electrolyte 26 should be added. The additional steps ofinserting electrode 28 and the plug 24 and attaching connector block 30and pin 32, may be performed in any convenient order, and may preceedthe impregnation of tube 20 with the ion-selective material. Because theelectrolyte chamber for the sensing electrode is fashioned integrallywith the ion-selective membrane, production of the electrode of FIG. 2is considerably simplified as compared to the prior art.

The effectiveness of ion-selective membranes and ion-sensing electrodesaccording to the present invention is demonstrated by the followingexamples:

EXAMPLE 1

A membrane selective for the ammonium ion (NH₄ ⁺) was produced asfollows:

Silicone rubber tubing of the type manufactured by Dow-Corning ChemicalCo., commercially available under the trademark Silastic® was soaked ina solution of 10 mg of ammonium ion-selective material, Nonactin, anantibiotic, available from Sigma Chemical Co. of St. Louis, Mo.,dissolved in 2.3 ml of methylene chloride, until the tubing swelled. Thetubing was removed from the solution and allowed to dry until itreturned to its original dimensions. Three such ammonium ion-selectivemembranes were incorporated in ion-sensing electrodes as illustrated inFIG. 2. The ion-sensing electrodes were placed in aqueous solutionshaving known molar concentrations of ammonium ions, along with areference electrode, and the voltage differences between the referenceand sensing electrodes were observed. The correlation of the knownmolarities of the ammonium ion with the measured voltage differences,illustrated in Table 1, indicates that the ion-sensing electrodes, wereeffective for their intended use.

                  TABLE 1                                                         ______________________________________                                                 Measured Voltages                                                    Molarity NH.sub.4.sup.+                                                                  Electrode 1 Electrode 2                                                                             Electrode 3                                  (M)        (mV)        (mV)      (mV)                                         ______________________________________                                        .1         150.1       148.7     140.1                                        .01        97.3        94.5      87.8                                          .001      43.9        44.9      34.5                                         E°  203.7       199.8     193.1                                        slope      53.1        51.9      53.3                                         r          1.0000      0.9997    1.000                                        ______________________________________                                    

EXAMPLE 2

A membrane selective for potassium ion (K⁺) was produced as follows:

A length of polyvinylchloride tubing was soaked in a solution of 10 mgof the potassium ion-selective material valimomycin dissolved in 2.5 mldipentyl phthalate (DPP) available from Eastman Kodak Chemical Co.,until the tubing swelled. The tubing was removed and dried for 16 hoursat 70° C. Two such potassium ion-selective membranes were incorporatedin ion-sensing electrodes as illustrated in FIG. 2. The ion-sensingelectrodes were placed in solutions having known concentrations ofpotassium ions, along with reference electrodes. The voltage differencesbetween the ion-sensing electrodes and the reference electrodes,illustrated in Table 2, indicate that the ion-sensing electrodes wereeffective for their intended use.

                  TABLE 2                                                         ______________________________________                                                     Measured Voltages                                                Molarity K.sup.+                                                                             Electrode 1                                                                             Electrode 2                                          (M)            (mV)      (mV)                                                 ______________________________________                                        1.0 × 10.sup.-3                                                                        -68.0     -71.6                                                4.0 × 10.sup.-3                                                                        -32.1     -37.4                                                6.0 × 10.sup.-3                                                                        -22.2     -26.5                                                E°      109.2     101.3                                                slope          59.1      57.7                                                 r              0.9999    0.9999                                               ______________________________________                                    

A similar procedure was used to fabricate potassium selective membranesfrom polyurethane tubing.

EXAMPLE 3

A membrane selective for the potassium ion (K⁺) was produced as follows:

A length of tubing extruded from the polyurethane available under thetrade designation Pellathane®, from Upjohn, was soaked in a solution of10.0 mg valinomycin and approximately 0.1 mg potassium tetraphenylborate dissolved in 2.0 ml acetone until the tubing swelled. The tubingwas removed from the solution and allowed to dry until the tubingreturned to its original dimensions. Two such potassium ion-selectivemembranes were incorporated in ion-sensing electrodes as illustrated inFIG. 2. The ion-sensing electrodes along with a reference electrode wereplaced in an aqueous solution having a known molar concentration ofpotassium ions, to which known increments of a 0.1M potassium solutionwere added, and the voltage differences between the reference andsensing electrodes were observed. The correlation of the knownmolarities of the potassium ions with the measured voltage differences,illustrated in Table 3, indicates that the ion-sensing electrodes wereeffective for their intended use.

                  TABLE 3                                                         ______________________________________                                                     Measured Voltages                                                Molarity K.sup.+                                                                             Electrode 1                                                                             Electrode 2                                          (M)            (mV)      (mV)                                                 ______________________________________                                        9.9 × 10.sup.-4                                                                        -44.9     -41.0                                                2.0 × 10.sup.-3                                                                        -32.9     -29.9                                                3.8 × 10.sup.-3                                                                        -20.1     -17.2                                                5.7 × 10.sup.-3                                                                        -12.2     -9.6                                                 9.1 × 10.sup.-3                                                                        -2.7      -0.8                                                 1.2 × 10.sup.-2                                                                        +3.2      +4.2                                                 E°      92.9      89.5                                                 slope          44.3      42.0                                                 r              0.9998    0.9998                                               ______________________________________                                    

EXAMPLE 4

A membrane selective for the chloride ion (Cl⁻) was produced as follows:

A length of silicone rubber tubing as described in Example 1, above, wassoaked in a solution of one part by volume of the chloride ion-selectivematerial methyl tricapryl ammonium chlroide, commercially available fromGeneral Mills Co. under the trade designation Aliquat® to two parts byvolume of Xylene. The tubing was soaked for about one day and thenvacuum dried. The Xylene was found to come off easily, but a surfaceresidue of Aliquat remained which was removed by washing with additionalXylene. Two such membranes were incorporated in ion-sensing electrodesas illustrated in FIG. 2. These sensing electrodes were placed inaqueous solutions having known concentrations of chloride ion, andtested as described in Examples 1 and 2 above. The results, illustratedin Table 4, indicate that the ion-sensing electrodes were effective fortheir intended use.

                  TABLE 4                                                         ______________________________________                                                     Measured Voltages                                                Molarity Cl.sup.-                                                                            Electrode 1                                                                             Electrode 2                                          (M)            (mV)      (mV)                                                 ______________________________________                                        .1             55.9      81.5                                                 .01            99.4      128.8                                                 .001          137.8     169.8                                                E°      15.8      38.9                                                 slope          -40.9     -44.2                                                r              .9994     .9992                                                ______________________________________                                    

EXAMPLE 5

A membrane appropriate for use in a pH sensing electrode was produced asfollows:

A length of silicone rubber tubing as described above was soaked in asolution of one part by volume of tridodecylamine to two parts by volumeof Freon until the tubing swelled. The tube was allowed to air dry untilit returned to its original dimensions. Three such membranes wereincorporated in ion sensing electrodes as illustrated in FIG. 2, using apH4 buffer containing NaCl as an electrolyte. These pH sensingelectrodes were placed in solutions having known pH, and tested asdescribed in the examples above. The results, illustrated in Table 5,indicate that the ion-sensing electrodes were effective for theirintended use.

                  TABLE 5                                                         ______________________________________                                        Measured Voltages                                                                    Electrode 1   Electrode 2                                                                             Electrode 3                                    pH     (mV)          (mV)      (mV)                                           ______________________________________                                        6      -133.3        -132.1    -144.6                                         7      -190.8        -184.8    -189.8                                         8      -241.6        -240.5    -242.8                                         E°                                                                            190.1         193.6     151.3                                          slope  54.1          54.2      49.1                                           r      .9996         .9949     .9990                                          ______________________________________                                    

FIG. 3 illustrates a side sectional view of a combination ion-sensingand reference electrode according to the present invention. Theion-selective membrane is fashioned from bi-lumen plastic tube 40, ofwhich the portion distal to dotted line 48 is impregnated with anion-selective material, using the process described above. The firstlumen 68 of tube 40 is filled with electrolyte 50, and acts as theelectrolyte chamber for the ion-sensing electrode. Second lumen 66 isfilled with electrolyte 52, and acts as the electrolyte chamber for thereference electrode. Electrolyte 52 is exposed to the sample solutionvia aperture 64. The distal ends of lumens 66 and 68 are sealed by plug46 which may be made of silicone rubber or other suitable material. Theproximal ends of lumens 66 and 68 are sealed by connector block 58,which may be made of epoxy or other suitable material. Mounted withinconnector block 58 are connector pins 60 and 62, which are crimped tothe proximal ends of electrodes 54 and 56. Electrodes 54 and 56 may besilver/silver chloride reference electrodes, well known to the art.Connector pins 60 and 62 may be made of any conductive metal such asstainless steel. Electrolytes 50 and 52 should contain silver andchloride ions, along with the ion to be detected. Because electrolyte 52is exposed to the sample solution, it is desirable that it be a gelelectrolyte. A preferred gel is discussed below.

The above electrode is fashioned by treating tube 40, distal to dottedline 48, as discussed in conjunction with FIG. 1 above, by dipping thedistal end of tube 40 into the solvent. Following evaporation of thesolvent, electrolyte 50 and 52 should be added. The additional steps ofinserting electrodes 54 and 56, attaching connector block 58, attachingconnector pins 60 and 62, inserting plug 46, and forming aperture 64 maybe performed in any convenient order, and may preceed the impregnationof tube 40 with the ion-selective material. Aperture 64 may be producedby simply placing a hole in a portion of outer tube wall 42, openinglumen 66 to the exterior of tube wall 42 and allowing free migration ofions from the solution to be tested into electrolyte 52. It is importantto note that aperture 64 is located proximal to dotted line 48, and thatplug 46 extends proximal to dotted line 48, thereby providing a sealintermediate the electrolyte contained within lumen 66 and the portionof tube 40 which has been impregnated with the ion-selective material.The seal is desirable to prevent migration of ions between the lumenswhich would interfere with the efficiency of the electrode. Because theelectrolyte chambers for both the sensing electrode and the referenceelectrode are fashioned integrally with the ion-selective membrane,production of the electrode of FIG. 3 is considerably simplified ascompared to the prior art.

EXAMPLE 6

A combination potassium slective electrode and reference electrode wasproduced as folows:

A length of bi-lumen silicone rubber tubing was soaked in a solution of10 mg valinomycin and 2.5 ml of Xylene until the tubing swelled. Thetubing was removed from the solution and allowed to dry until the tubingreturned to its original dimensions. Combination electrodes asillustrated in FIG. 3 were fabricated from this tubing. The combinationelectrodes were placed in aqueous solutions having known molarconcentrations of potassium ions, and the voltage differences betweenthe potassium sensing electrode and the reference electrode wereobserved. The correlation of the known molarities of the potassium ionwith the measured voltage differences, illustrated in Table 6 indicatesthat the combination electrodes were effective for their intended use:

                  TABLE 6                                                         ______________________________________                                                       Measured Voltages                                              Molarity K.sup.+                                                                       Electrode 1 Electrode 2                                                                             Electrode 3                                    (M)      (mV)        (mV)      (mV)                                           ______________________________________                                        1 × 10.sup.-3                                                                    -152.1      -108.6    -118.4                                         4 × 10.sup.-3                                                                    -117.7      -74.3     -84.1                                          6 × 10.sup.-3                                                                    -108.2      -64.0     -74.6                                          E°                                                                              38.9        5.8       5.4                                            slope    56.7        57.2      56.5                                           r        0.9999      0.9999    0.9999                                         ______________________________________                                    

FIG. 4 illustrates a side sectional view of a combination ion-sensingand reference electrode according to the present invention. Like theelectrode of FIG. 3, this electrode is fashioned from a bi-lumen plastictube 70. That portion of tube 70 which is distal to dotted line 78 isimpregnated with an ion-selective material.

Only the distal end of lumen 98, which serves as the electrolyte chamberfor the ion-sensing electrode, is sealed by means of plug 76, which maybe made of silicone rubber or other suitable plastic. The distal end oflumen 96 remains open to the exterior of tube 70, via aperture 94. Lumen96 serves as the electrolyte chamber for the reference electrodeportion. Electrodes 84 and 86, connector block 88, and connector pins 90and 92 are identical to the corresponding elements in FIG. 3, above.

The above electrode is fashioned by treating bi-lumen tube 70 distal todotted line 78 as discussed in conjunction with FIG. 1 above. This maybe accomplished by dipping only that portion of tube 70 distal to dottedline 78 in the solvent. Following the evaporation of the solvent,electrolyte 80 and 82 should be added. Electrolyte 80 and 82 correspondto electrolyte 50 and 52 of FIG. 3. The additional steps of insertingelectrodes 84 and 86, attaching connector block 88, inserting plug 76and attaching connector pins 90 and 92, may be performed in anyconvenient order, and may proceed the impregnation of tube 70 with theion-selective material. It is important to note that lumen 96 does notextend distal to dotted line 78. As such, no portion of inner wall 74which is impregnated with the ion-selective material, is exposed tolumen 96, preventing transfer of ions between lumens. Bi-lumen tube 70may be conveniently fabricated by removing a portion of the outer tubewall surrounding lumen 96, intermediate the distal end of tube 70 and apoint proximal to dotted line 78. Because the electrolyte chambers forboth the sensing and reference electrodes are fashioned integrally withthe ion-selective membrane, production of the electrode of FIG. 4 isconsiderably simplified as compared to the prior art. Further, theremoval of a portion of the outer wall, surrounding lumen 96 to preventselective ion transfer between lumens 96 and 98 is believed to be botheasier to accomplish and more effective than the structure of FIG. 3,which requires that plug 46 tightly seal lumen 66 distal to aperture 64.Leakage of this seal could result in selective ion transfer betweenlumens 68 and 66. There is no corresponding seal in the structure ofFIG. 4, and possibilities of malfunction of the electrode due to sealleakage are therefore believed to be reduced.

FIG. 5 illustrates a side sectional view of a flow through ion-sensingelectrode. The ion-selective membrane of the electrode is fashioned fromplastic tube 102, which has outer wall 106 impregnated with anion-selective material, as described above. Tube 102 also serves as thesample tube for the electrode. Located coaxially around tube 102 isouter tube 100, which may be conveniently made of any appropriateplastic. The proximal and distal ends of tube 100 are sealed around tube102 by means of sealing disks 110 and 112, respectively. That portion oflumen 126 of tube 100 intermediate tube wall 104 of tube 100 and tubewall 106 of tube 102 is filled with electrolyte 114. Electrolyte 114corresponds to electrolyte 26 of FIG. 2. The proximal and distal ends oftube 102 are coupled to fluid couplings 118 and 116, respectively. Fluidcouplings 116 and 118 may be any appropriate fluid couplings adapted foruse with plastic or rubber tubing. Electrode 122 and connector pin 124are identical to the corresponding elements in the figures above.Securing connector pin 124 to tube 100 is tubular sleeve 120, which maybe fashioned of an appropriate plastic or other material.

The above electrode is fashioned by first treating tube 102 as discussedin conjunction with FIG. 1 above. The entire tube, or only a portion maybe treated. Following evaporation of the solvent, tube 100 and sealingdisks 110 and 112 are added. The electrolyte 114 may then beconveniently added, however, there is no criticality in the order of thesteps. The steps of inserting electrode 122 and attaching connector pin124 and tubular sleeve 120 are similarly not critical to the manufactureof this device. Because the sensing membrane is formed integrally withthe sample tube of the flow through electrode, production of theelectrode of FIG. 5 is considerably simplified as compared to the priorart.

FIG. 6 illustrates a side sectional view of a preferred embodiment of aflow-through ion-sensing electrode according to the present invention.The ion-selective membrane of the electrode is fashioned from bi-lumenpolymer plastic tube 130, which has at least inner wall 134 impregnatedwith an ion-selective material, as described above. Lumen 154 of tube130 is filled with electrolyte 146, which corresponds to electrolyte 26of FIG. 2. Sealing the proximal and distal ends respectively of lumen154 are plugs 136 and 138, which may be conveniently fashioned ofsilicone rubber or other suitable material. Lumen 140 of tube 130 servesas the sample tube for the electrode. Coupled to the proximal and distalends of lumen 140 are fluid couplings 142 and 144 respectively. Fluidcouplings 142 and 144 are identical to the corresponding structuresshown in FIG. 5. Electrode 152, connector pin 150, and tubular sleeve148 correspond to the identically named structures in FIG. 5 above.

The above electrode is fashioned by first treating a bi-lumen tube 130as discussed in conjunction with FIG. 1 above. The entire tube may betreated, or only a portion thereof. Following the evaporation of thesolvent, electrolyte 146 should be added. The additional steps ofinserting plugs 136 and 138, inserting electrode 152, attaching circularsleeve 148 and connector pin 150, and inserting fluid couplings 142 and144 may be performed in any convenient order, and may preceed theimpregnation of tube 130 with the ion-selective material. Because theelectrolyte chamber and the sample tube of this electrode are fashionedintegrally with the ion-selective membrane, production of the electrodeof FIG. 6 is considerably simplified as compared to prior artelectrodes.

FIG. 7 illustrates a side sectional view of a combination of ion-sensingand reference flow-through electrodes according to the presentinvention. The ion-selective membrane of the electrode is fashioned frombi-lumen plastic tube 160, of which at least inner wall 164 isimpregnated with an ion-selective material, distal to dotted line 174.The proximal and distal ends of lumen 196 of tube 160 are sealed bymeans of plugs 166 and 168 respectively. In addition, lumen 196 issealed centrally by means of plug 172. The portion of lumen 196 distalto plug 172 serves as the electrolyte chamber for the ion-sensingelectrode. That portion of lumen 196 located proximal to plug 172 servesas the electrolyte chamber for the reference electrode. Inner wall 164is provided with aperture 198, exposing electrolyte 182 to lumen 170,which serves as the sample tube for the electrode. Electrolyte 180 and182 correspond to electrolyte 50 and 52 of FIG. 3. Coupled to theproximal and distal ends of lumen 170 are fluid couplings 178 and 176,respectively, which are identical to the corresponding structures inFIGS. 5 and 6. Similarly, electrodes 184 and 186, tubular sleeves 192and 188, and connector pins 190 and 194 are identical to thecorresponding structures having the same names in FIGS. 5 and 6.

The above electrode is fashioned by treating a bi-lumen tube 160 asdiscussed in conjunction with FIG. 1 above. Only the portion of the tubedistal to dotted line 174 should be impregnated with the ion-selectivematerial. Following evaporation of the solvent, electrolyte 180 and 182should be inserted in lumen 196. The additional steps of inserting plugs166, 168 and 172, fluid couplings 176 and 178, connector pins 190 and194, circular sleeves 188 and 192, and electrodes 184 and 186, alongwith the step of cutting aperture 198, may be performed in anyconvenient order and may proceed the impregnation of tube 160 with theion-selective material. Because the electrolyte chambers for both thesensing and reference electrodes of the present invention, along withthe sample tube, are all constructed integrally with the ion selectivemembrane, production of the electrode of FIG. 7 is considerablysimplified as compared to the prior art. It is important to note thatseal 172 is located proximal to dotted line 174, so that ion transferbetween the sensing and reference chambers is precluded.

With regard to the electrodes of FIGS. 6 and 7, the construction of suchelectrodes, using commercially available bi-lumen plastic tubingprovides the opportunity for arranging any number of ion-sensing andreference electrodes within a single unified tubular structure. Such amultiple electrode structure is believed within the scope of the presentinvention.

An electrolyte gel appropriate for use in all of the above embodiments,but especially useful in the embodiments of FIGS. 3, 4 and 6, maycomprise a polyvinyl alcohol gel comprising (by weight) 7%polyvinylalcohol, 92.5% water, and 0.5% concentrated (18M) sulfuric acidas a polymerizing agent. The gel should be saturated with silverchloride, and be provided with electrolytes including the ion to besensed. For example, electrolyte levels of 0.15M NaCl and 0.1MKCl wouldbe appropriate in a chloride ion sensing electrode. The gel may bestabilized by including 0.025% (be weight) gluteraldehyde as a crosslinking agent. Such a gel is a stable, non-swelling, non-erroding gel,suitable for use with the above described electrodes. In addition, sucha gel is capable of withstanding flexing without crumbling. Thesecharacteristics are especially desirable in a gel used with the aboveelectrodes, because the gel is exposed directly to the sample solutionin the reference electrode portions of the electrodes shown in FIGS. 3,4, and 7.

Although several embodiments of the invention have been disclosedherein, it will be understood that the embodiments disclosed may besubjected to various changes, modifications and substitutions withoutdeparting from the spirit and scope of the invention as defined by theappended claims.

We claim:
 1. A method for producing a flow-through ion sensingelectrode, comprising the following ordered steps:dissolving an ionselective material in a solvent which is a swelling agent for a plastic;subsequently soaking in said solvent at least a portion of a plastictube having a proximal end, a distal end, an outer wall, a first lumen,a second lumen, and an inner wall separating said first and secondlumens, until said portion of said tube swells, said portion includingat least part of said inner wall; subsequently drying said tube toremove said solvent; and subsequently filling only the first lumen ofsaid tube with an electrolye; said method further comprising thefollowing non-ordered steps which may be performed in any convenientorder relative to said ordered steps: sealing said first lumen distal toat least a part of said portion of said tube and sealing said firstlumen proximal to at least a part of said portion of said tube; andinserting an electrode into the first lumen of said tube intermediatesaid first and second sealing means.
 2. A method for producing an ionsensing electrode according to claim 1, wherein said soaking stepconsists of soaking only a portion of said tube distal to a first pointintermediate the proximal and distal ends of said tube, and furthercomprising the following non-ordered steps:providing an aperture in theinner wall of said tube at a second point proximal to said first pointopening the first lumen of said tube to the second lumen of said tube;sealing the first lumen of said tube intermediate said first and secondpoints; and inserting a second electrode in the first lumen of saidtube.
 3. A method for producing a flow through ion sensing electrodefrom a length of tubing having a first lumen, a second lumen, an innerwall separating said first and second lumens, an outer wall, a proximalend and a distal end, fabricated of a swellable solid plastic,comprising the steps of:dissolving an ion selective material in asolvent which is a swelling agent for said swellable solid plastic;soaking a portion of said plastic tubing including at least part of saidinner wall in said solvent in which said ion selective material has beendissolved until said portion of said tubing swells; drying said swelledtubing until said portion has returned to its original size and saidsolvent is removed from said tubing; and after said drying step,inserting an electrode into said first lumen of said tubing andinserting an electrolyte only in said first lumen of said tubing incontact with said electrode.
 4. A method according to claim 3 whereinsaid soaking step consists of soaking only a portion of said tubingdistal to a first point intermediate said proximal and distal ends ofsaid tubing, and further comprises the steps of:providing an aperture insaid inner wall of said tubing at a second point proximal to said firstpoint; sealing said first lumen of said tube at a third pointintermediate said first and second points; and wherein said insertingstep comprises inserting a first electrode into said first lumen of saidtubing proximal to said third point and a second electrode into saidfirst lumen distal to said third point and inserting electrolyte intosaid first lumen of said tubing both proximal and distal to said thirdpoint and in contact with said first and second electrodes.